The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art to the present invention.
Variations in chromosome structure involve changes in parts of chromosomes rather than changes in the number of chromosomes or sets of chromosomes in the genome. There are four common types of mutations: deletions and duplications (both of which involve a change in the amount of DNA on a chromosome), inversions (which involve a change in the arrangement of a chromosomal segment), and translocations (which involve a change in the location of a chromosomal segment). All four classes of chromosomal structure mutations are initiated by one or more breaks in the chromosome. If a break occurs within a gene, then a gene mutation has been produced, the consequence of which depends on the function of the gene and the time of its expression. Wherever the break occurs, the breakage process leaves broken ends, which may adhere to other broken chromosome ends or the normal ends of other chromosomes.
Reciprocal and Robertsonian translocations are the most frequently occurring types of translocations. Reciprocal translocations usually involve a two-way exchange between different chromosomes. The chromosomes break apart and segments below the break points swap positions. If the event is balanced, no net gain or loss of genetic material results and the individual is usually phenotypically unaffected if no genes are disrupted.
Robertsonian translocations occur when two chromosomes fuse at the centers and essentially combine into one. Most of the genetic material remains from both chromosomes. As in balanced reciprocal translocations, the carrier may be normal, but produce genetically unbalanced gametes. Most progeny originating from unbalanced gametes do not survive and a miscarriage occurs during early pregnancy. If the carrier is fertile and progeny survive, various defects could occur. One Robertsonian translocation results in the fusion of chromosomes 14 and 21. Resulting progeny may inherit three copies of chromosome 21 which causes Down's syndrome.
Genetic abnormalities such as duplication, deletion, chromosomal translocation, and point mutation often lead to pathological conditions. Some diseases, such as cancer, are due to genetic abnormalities acquired in a few cells during life, while in other diseases the genetic abnormality is present in ail cells of the body and present since conception.